Download 1 – Structure and stocks of coral reef fish communities

Aquat. Living Resour. 13 (2) (2000) 65−76
© 2000 Ifremer/Cnrs/Inra/Ird/Cemagref/Éditions scientifiques et médicales Elsevier SAS. All rights reserved
S0990744000001455/FLA
Fish stock assessment of the northern New Caledonian lagoons:
1 – Structure and stocks of coral reef fish communities
Yves Letourneur(a, b*), Michel Kulbicki(b), Pierre Labrosse(c)
(a)
Station marine d’Endoume, centre d’océanologie de Marseille, université de la Méditerannée, UMR 6540 Dimar,
rue de la Batterie des Lions, 13007 Marseille, France
(b)
(c)
Centre IRD, BP A5, 98848 Nouméa cedex, New Caledonia
Secretariat of the Pacific Community, Reef fisheries assessment and management section, BP D5,
98848 Nouméa cedex, New Caledonia
Received 27 August 1999; accepted 8 March 2000
Abstract — Lagoon fish in New Caledonia are mainly caught by artisanal fisheries and subsistence fishing. Reef fish are the major
component of this catch. The present study aimed at estimating these reef fish standing stocks and at finding the main factors influencing the
distribution of these fish. Sampling of 904 stations was stratified according to three zones (north, east and west) and three reef types (barrier,
intermediate and fringing). Fish communities exhibited strong heterogeneity in their distribution, showing higher biomass (maximum of
447 g·m–2) and total standing stock (43 000 tonnes) in the north zone than in the east and west zones. Similarly, observed patterns were
dependent on reef types: higher biomass and total standing stock being observed on barrier reefs than on intermediate or fringing
reefs. The total standing stocks, which were about 65 000 t, were mainly composed of herbivorous fish families such as the Acanthuridae
and Scaridae. The differences in the patterns of distribution of species, individuals and standing stocks between reef types may be explained
by variations in terrestrial influences and reef morphology, whereas differences among zones were most likely due to accessibility of fishing
areas and fishing pressure. The latter is almost non-existent in the north zone, which can thus be considered to be almost unexploited
commercially. This most likely explains the high proportion, 77 %, of long-lived species in the biomass of this zone. The results might
have implications in management of reefs elsewhere in the South Pacific, for which similar data are only scarcely available.
© 2000 Ifremer/Cnrs/Inra/Ird/Cemagref/Éditions scientifiques et médicales Elsevier SAS
Reef fish / commercial demersal fish / population structure / New Caledonia / SW Pacific Ocean
Résumé — Estimation des stocks de poisson des lagons de Nouvelle-Calédonie : 1 – Structure et stocks des communautés des
poissons de récifs. Les poissons de lagon de Nouvelle-Calédonie sont exploités par les pêcheries artisanales, dont les poissons de récifs
constituent la majeure partie de ces captures. Ce travail vise à estimer les stocks de ces poissons récifaux et à déterminer les principaux facteurs
qui influencent leur distribution. L’échantillonnage de 904 stations a été stratifié en fonction de trois zones (nord, est et ouest) et de trois types
de récif (barrière, intermédiaire et frangeant). Les peuplements de poissons ont montré une très forte hétérogénéité dans leur répartition de
la biomasse et des stocks. Dans la zone nord, ces valeurs ont été bien supérieures (jusqu’à 447 g·m–2, et stocks de 43 000 tonnes) à celles des
zones est et ouest. La distribution de ces peuplements est également fonction du biotope considéré, de plus fortes densités et biomasses étant
relevées sur les récifs barrières, par contraste avec les récifs intermédiaires et frangeants. La majeure partie du stock total, qui est de l’ordre
de 65 000 t, est constituée d’espèces herbivores d’Acanthuridés et de Scaridés. Les principales causes des différences observés dans la
répartition des espèces et des individus, ainsi que des stocks entre types récifaux seraient liées à des différences d’apports terrigènes et de
morphologie récifale, tandis que les différences relevées entre les trois zones sont davantage liées à l’accessibilité des ressources et à la
pression de pêche. Cette dernière est très faible dans la zone nord, qui peut être considérée comme inexploitée. Cela expliquerait la forte
proportion, 77 %, des espèces à longue espérance de vie de la biomasse totale de cette zone. La gestion des pêcheries artisanales dans
l’ensemble du Pacifique Sud, où les données sont rarement disponibles, implique la prise en compte de ces observations.
© 2000 Ifremer/Cnrs/Inra/Ird/Cemagref/Éditions scientifiques et médicales Elsevier SAS
Poissons de récifs / poissons démersaux commerciaux / structure de population / Nouvelle-Calédonie / océan Pacifique
* Corresponding author: [email protected]
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66
1. INTRODUCTION
In coral reef ecosystems, fish are usually the most
important resource, especially for developing countries in the Pacific and Caribbean where they may
constitute the major protein supply. Unfortunately,
most coral reefs across the world are increasingly
subjected to various types of human pressures [8, 14,
47], which almost always result in a decline in reef
health [40]. Despite the lack of data on pristine situations, it is generally accepted that an increase in fish
consumption, mainly linked to demography, results in
a concomitant increase in fishing pressure on the coral
reefs of various geographical areas [16, 17, 19, 41,
44]. In several cases, overfishing has occurred, or will
be reached in the very near future, and is considered
one of the most serious threats to coral reefs [37, 38,
41]. However, contrary to general opinion, studies on
the effects of fishing on fish communities are scarce
and are usually too recent to allow a good understanding of these phenomena [15, 16, 19, 43, 44]. Indeed,
fishing effects are widely treated as equivalent to
overfishing and thus many studies of fishing effects
have been based upon studies of localized catastrophic
events rather than changes linked to fishing under
normal conditions [17]. In addition, most studies have
compared variations in density and biomass among
sites and years to assess fishing effects on the ichthyofauna, whilst other basic data from the same areas,
such as stock estimates, catches or CPUEs, are seldom
simultaneously available. Therefore, relationships between ecological and fisheries-dependent data are
difficult to establish. The description of such relationships is nevertheless needed to understand how fisheries characterized by complex fish communities may
evolve.
We conducted a stock assessment of lagoonal and
reefal demersal fish of commercial interest at the
request of the Northern Province of New Caledonia in
the SW Pacific Ocean. In this province, human density
is low, 4.2 inhabitants·km–2, especially when compared to those found on other Pacific islands: 34
inhabitants·km–2 in Palau, 132 in Tonga, 271 in Guam
and 505 in Nauru [9]. If we consider this low density,
the artisanal nature of the local fisheries and the very
large area (15 000 km_) of the lagoons of this province, we can make the hypothesis that, overall, local
fish communities of commercial interest are not substantially depleted by fishing activities. However, the
fish communities of this province have never been
studied, except for two specific and spatially limited
works focusing on bait-fishes for tuna fisheries [7] and
on soft-bottom fishes [50].
The main objectives of the whole study (e.g. this
paper and the two companion ones [22, 27]) were as
follows: to define the structure of fish communities at
different spatial scales (whole province, three main
zones, three main biotopes); to provide estimates of
both total and exploitable stocks for the whole community and major fish families; and to analyse the
Y. Letourneur et al.
relationships between field data and the existing and
expected socio-economic aspects of the fishery. This is
the only way to test our hypothesis that these fish
communities are not overfished. The three major
biotopes (e.g. coral reefs, near-reef areas and lagoon
bottoms) examined in this whole study display considerable differences in their ecological characteristics.
This is the reason why they were censused with
different methods.
The present paper focuses on reef fish, which were
studied by visual censuses, whereas the near-reef and
the lagoon bottom areas, which were investigated with
line-fishing techniques, are presented in the first companion article [22]. Field techniques provide valuable
ecological information but give little information on
the relationships between human and fish populations.
Despite their fundamental importance, primarily for
island nations, the social and economic aspects sensu
lato of reef fisheries have been almost systematically
neglected or at least underestimated [36]. This lack of
information on these aspects may explain many failures in management options on coral reef areas [1, 9,
18, 35]. This topic was studied in the second companion article [27], particulary because in New Caledonia
there is strong customary tradition that greatly influences fishing activities. The whole study conducted
over a period of two and a half years constitutes one of
the largest works of this kind ever conducted in the
Indo-Pacific region on multispecific lagoonal and reefal fish communities with different techniques and at
such a large spatial scale.
2. MATERIALS AND METHODS
2.1. Study area and sampling zones
New Caledonia is located in the south-western
Pacific Ocean (figure 1), and coral reefs are well
developed around this island. The present work was
conducted on the coral reefs of the Northern Province
of New Caledonia (figure 1). Three main zones were
defined according to natural partitions: the north, the
west and the east zones. The area studied covers
approximately 10 000 km2 of lagoonal waters, including about 700 km2 of coral inner reef slopes. The
northern limit of this study was 19°30’S.
In each zone, the three existing reef types were
sampled. There were, from land outwards to open
ocean: the coastal fringing reefs, the intermediate reefs
(e.g. the reefs close to lagoonal islets), and the barrier
reefs. A regular sampling strategy was adopted on the
entire reef system of the area covered. Stations were
chosen approximately 0.6 nautical miles apart from
one another for each reef type. The sampling effort in
each zone and reef type was thus proportional to the
length of the coral reefs [28]. In addition, the number
of stations was adapted to the estimated surface of reef
slopes at the sampled depths (2–8 m on average) [28],
resulting in a higher number of samples in areas with
low reef slopes than in those with abrupt reef slopes.
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New Caledonia: 1 – Coral reef fish
67
Figure 1. Location of the studied lagoons of the Northern Province of New Caledonia, in the south-western Pacific Ocean.
Sampling was conducted between February and April
1995 using the R/V ‘Alis’ for the north zone, between
July 1995 and January 1996 using a smaller boat and
motorized canoes for the west zone, and between
March 1996 and February 1997 for the east zone, also
using a small boat and motorized canoes. A total of
904 stations were investigated on coral reefs during
the whole study.
2.2. Fish sampling technique
In a multispecific context, it is often difficult to
define what is precisely a commercial species. In New
Caledonia, almost all species belonging to the major
coral reef fish families of interest (e.g. Serranidae,
Lutjanidae, Lethrinidae, Scaridae, Acanthuridae and
Siganidae) are caught. In addition, several species
belonging to the families Carangidae, Haemulidae and
Labridae are also regularly targeted. All these species
were therefore censused and considered as commercial
for the purpose of our study. They were studied by
underwater visual censuses using the line-transect
method [5] in the 2–8-m depth range. This census
method is derived from terrestrial procedures. Its
precision and accuracy for underwater work has been
previously demonstrated [11, 21] and utilized by several authors [20, 23, 32, 34]. On each station, two
divers censused fishes along a 50-m transect, one on
each side of the transect line. The observers counted
all commercial fish, and recorded the estimated perpendicular distance between the fish and the transect
line. Density and biomass estimates (see below) are
based on the detectability functions of species. These
parameters are usually better estimated for commercial
fish species by the method we used than by fixedwidth transects or fixed-radius point counts [21]. The
size of fish (total length) was estimated using 1-cm
classes for fish ranging from 1 to 10 cm, 2-cm classes
for fish ranging from 10 to 30 cm, 5-cm classes for fish
from 30 to 60 cm, and 10-cm classes for fish larger
than 60 cm. Increasing size classes is recommended
for increasing fish size [3]. If fishes were in a school or
a group, their number was estimated and the perpendicular distance of the closest fish and the furthest fish
was recorded. Despite usual bias of visual census, this
method is particularly efficient in biotopes where
human visits (diving, spearfishing, etc.) are rare, because the behaviour of fish remains natural [21].
2.3. Demographic structures
Each fish may be assigned to one demographic
category, according to the main characteristics of the
species to which it belongs. The knowledge of the
biological and ecological characteristics for the species of New Caledonian reefal and lagoonal waters [24] allowed us to define three main demographic
categories: species with a short life-span (characterized by fast growth, small size, high gonado-somatic
index and early reproduction), species with a long
life-span (characterized by slow growth, large size,
low gonado-somatic index and late reproduction), and
species with intermediate characteristics [24]. The demographic structure, expressed in terms of density and
biomass, is a useful parameter for understanding
differences in fish community patterns, even when
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68
Y. Letourneur et al.
restricted to only commercial species. Indeed, this
allows prediction of the major descriptors of the
response of fish communities to potential disturbances
which might affect them [26, 31].
2.4. Data analyses
Data of the two observers cannot be used as replicates because the environment within a station may be
heterogeneous (for example, when the census was
carried out near the basal part of the reef slope, one
diver may have counted on the ‘reef side’ of the
transect and the other on the ‘sand side’). Data were
thus pooled for each transect. Among the many density
estimators that may be chosen [6], the most robust for
this type of data is [51]:
Di = 共 2L 兲
−1
兺
共 ni /di 兲
where Di is the density of species i (fish·m–2), L is the
transect length (50 m), ni is the abundance of species i,
di is the average distance of species i to the transect
(m). Average distance for species i was calculated as
follow:
di = 共 n i 兲
−1
兺共 n d 兲
j
j
where nj is the number of fish of species i observed at
occurrence j, and dj is the distance of fish of species i
to the transect at occurrence j.
The weight of fish was calculated from length/
weight relationships as previously defined [33]. Biomass can be calculated in a similar way to density:
Bi = 共 2L 兲
−1
photographs from the Institut Géographique National.
The stocks were estimated by:
Si = Ai × Bi
where Si is the stock for zone or reef type i (tonne), Ai
is the surface area of zone or reef type i (km2), and Bi
is the mean biomass in zone or reef type i (g·m–2,
which is equivalent to tonne·km–2).
3. RESULTS
3.1. Spatial structure
of the whole commercial ichthyofauna
The ANOVAs indicated that the factors ‘zone’ and
‘reef type’ were statistically significant on the patterns
found (table I). The mean species richness and mean
biomass per census were significantly higher in the
north zone than in the two others (table II). The mean
density was highest in the west zone, and lowest in the
east zone (table II), but the difference between these
values was not statistically significant (table I). The
highest values of mean species richness, density and
biomass were found on barrier reefs (table II), and
patterns were significant except for densities between
barrier and intermediate reef (table I). There were also
significant interactive effects between zones and reef
types (table I), mainly underlining that differences
between reef types for the three descriptors tested
(species richness, density and biomass) within each
zone were, globally, similar to differences for all zones
pooled together.
兺共 w /d 兲
i
–2
i
where Bi is the biomass (g·m ), and wi is the weight
of species i (g).
A principal component analysis (PCA) was applied
for the 904 stations to the three ‘global’ fish descriptors
(i.e. species richness, density and biomass) which were
normalized due to their different natures. The heterogeneity of dispersion was tested with random permutations of stations between the nine areas (e.g. 3 zones
× 3 reef types) [42]. The data met the Levene’s test
criteria (homogeneity of variances) [48], and two-way
ANOVAs (zone × reef type) were then performed on
the different types of data (species richness, density
and biomass). Reef types were considered as nested
within the zones. A posteriori differences among zones
were tested by Scheffé tests. If a significant nested
effect is detected, it is not possible to use post hoc tests
to assess the global reef type effect (e.g. for all zones
pooled together). However, in this case it is possible to
test the reef type effect within each zone, using a
posteriori SNK tests. The surface areas of the studied
sites (i.e. the slopes of the three reef types and the three
zones) were obtained with both marine maps from the
Mission Océanographique du Pacifique and aerial
Table I. Summary of ANOVAs involving species richness, density
and biomass of the total ichthyofauna. In each case, the first line
indicates the result of the ANOVA and the second the post hoc planned
comparison (Scheffé tests). For reef type effect within zone, SNK tests
were used for pairwise comparisons among treatments. N, north zone;
W, west zone; E, east zone; B, barrier reef; I, intermediate reef; F,
fringing reef; Fr, Français’ barrier reef; Co, Cook’s barrier reef; and Li,
lagoonal islands.
Source of variation
Species
richness
Density
Biomass
ns
Reef type effect
P < 0.0001
N>E>W
P < 0.0001
P < 0.0001
N>W>E
P < 0.0001
Reef type effect
within zone
North
West
East
ns
B>I>F
B=I>F
Zone effect
P = 0.0051
B=I>F
Fr = Co > Lia
B>F>I
B>I>F
a
In the north zone, both the Français’ and the Cook’s reefs are barrier
reefs. The term ‘lagoonal islands’ includes fringing reefs (Bélep
Islands) and also a few intermediate ones (see figure 1).
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New Caledonia: 1 – Coral reef fish
69
Table II. Total species richness, mean species richness, mean density (× 10–4 indivuals·m–2) and mean biomass (g·m–2) per census on the different
reef types of each major zone sampled. Values in parentheses are confidence intervals at α = 5 % level.
No. of stations
Total no. of species
Mean species richness
Mean density
Mean biomass
60
76
38
174
143
159
131
187
28.2 (2.0)
27.7 (2.1)
30.4 (3.2)
28.4 (1.4)
491 (70)
520 (89)
693 (108)
541 (61)
447 (103)
231 (43)
390 (87)
339 (68)
109
110
68
287
193
174
144
226
27.3 (1.6)
20.6 (1.8)
17.2 (2.0)
22.4 (1.1)
757 (55)
635 (37)
436 (40)
622 (36)
343 (35)
167 (20)
270 (28)
258 (21)
126
127
190
443
187
204
210
252
28.0 (1.6)
26.3 (1.8)
20.7 (1.3)
24.4 (1.0)
459 (66)
569 (97)
445 (82)
476 (58)
196 (33)
175 (23)
115 (18)
158 (16)
333
257
314
904
244
238
220
264
28.1 (0.9)
24.0 (1.3)
21.1 (1.1)
24.5 (0.7)
578 (82)
567 (94)
438 (78)
556 (65)
309 (68)
184 (36)
152 (34)
259 (63)
a
North zone
Français’ reef
Lagoonal islands
Cook’s reef
Total
West zone
Barrier reef
Intermediate reef
Fringing reef
Total
East zone
Barrier reef
Intermediate reef
Fringing reef
Total
All zones
Barrier reef
Intermediate reef
Fringing reef
Total
a
In the north zone, both the Français’ and the Cook’s reefs are barrier reefs. The term ‘lagoonal islands’ includes fringing reefs (Bélep Islands) and
also a few intermediate ones (see figure 1).
The PCA also revealed striking differences in communities (figure 2). The test of random permutations
was highly significant (P < 0.0005), underlining that
fish communities were significantly different between
the nine areas studied.
The short-lived species represented more than one
quarter of the fish density in each zone, ranging from
26.4 % in the north zone to 36.7 % in the west zone
(table III). Long-lived species were also abundant,
mainly in the north zone where they constituted more
than one third of the total density. The most abundant
species in each zone belonged to the intermediate
life-span categories, constituting up to 50.4 % of total
density in the east zone (table III). A v2 test indicated
that the demographic structures, expressed in terms of
density, were significantly different between zones (P
< 0.05). Patterns were different in terms of biomass,
mainly because long-lived species represented a more
important part of the fish communities of commercial
interest than for densities (table III). These species
represented about half of the total biomass in the east
zone, but were as high as 77.1 % of the total biomass
in the north zone. Patterns among zones, in terms of
biomass, were significantly different (v2 test; P <
0.001).
3.2. Spatial distribution
of fish families and species
Figure 2. Plot of the first factorial plane of the PCA monitored on the
normed data of species richness, density and biomass of the commercial fish from the 904 coral reef stations. Only the position of the
centres of gravity are indicated as follows: EB, east zone barrier reefs;
EI, east zone intermediate reefs; EF, east zone fringing reefs; NB,
north zone barrier reefs; NI, north zone intermediate reefs; NF, north
zone fringing reefs; WB, west zone barrier reefs; WI, west zone
intermediate reefs; WF, west zone fringing reefs.
Patterns of density displayed marked differences
according to families and zones (figure 3). In the west
zone, Lutjanidae (0.131 individuals·m–2) and Scaridae
(0.195) were significantly more abundant (SNK test, P
< 0.05) than in the two other zones. A similar pattern
was found in the east zone for Lethrinidae (0.043
individuals·m–2) and Siganidae (0.055); whereas
Acanthuridae were significantly (SNK test, P < 0.01)
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70
Y. Letourneur et al.
Table III. Demographic structure of the commercial fish communities in the three main zones, expressed (A) in percentage of density
and (B) in percentage of biomass (all reef types pooled).
North
West
East
(A)
Short life-span
Intermediate life-span
Long life-span
26.4
39.1
34.5
36.7
43.7
19.6
28.3
50.4
21.3
(B)
Short life-span
Intermediate life-span
Long life-span
4.9
18.0
77.1
14.0
24.5
61.5
10.9
36.4
52.7
more abundant in the north zone (figure 3). Acanthuridae, Scaridae and Lethrinidae displayed increasing
densities from fringing to barrier reefs. An opposite
trend was found for Siganidae, whereas Lutjanidae
were significantly more abundant (SNK test, P < 0.01)
on intermediate reefs (figure 3). The total biomass was
significantly lower in the east zone than in the two
other zones (SNK test, P < 0.05), and the magnitude of
the between zone differences was more marked than
for densities (figure 4). This indicates that fish reached
larger mean sizes in the north zone [32]. Similar trends
were observed for most families, although differences
between east and west zones were significant only
for Lutjanidae and Acanthuridae (P < 0.05). Biomass
was particularly high in the north zone for Acanthuridae (105 g·m–2), Scaridae (103) and, to a lesser
extent, Serranidae (25). As for density, total biomass
increased from fringing reefs to barrier reefs (figure 4).
This pattern was also similar for most of the other
families, except for Siganidae and Lutjanidae. Siganids displayed lower biomass on barrier reefs while
lutjanids showed no significant pattern (SNK test, P >
0.05) among reef types.
Not surprisingly, most species displayed a pattern of
distribution similar to the family to which they belong.
In most cases, mean weight of individuals, density
and/or biomass were significantly higher in the north
zone than in the two others, the west zone displaying
intermediate values, such as for Plectropomus leopardus, Lutjanus bohar, Cheilinus undulatus, Scarus
microrhinos and Naso unicornis (table IV). Some
other species displayed higher values for these descriptors in the west zone, such as Siganus lineatus.
Lethrinus nebulosus was the only species to show
higher density in the east zone, but this difference was
not statistically significant (P > 0.05) (table IV).
Differences among reef types were also marked
(table IV), thus highlighting an important land/ocean
gradient for most species. Mean weight of individuals,
density and biomass were higher on barrier reefs than
on intermediate and coastal fringing reefs, for L.
bohar, C. undulatus and N. unicornis. Some species
had their highest descriptor values on fringing reefs,
such as Siganus lineatus for density, and others on
intermediate reefs, such as P. leopardus for biomass,
whereas their mean weight was significantly higher (P
< 0.05) on barrier reefs (table IV).
3.3. Standing stock estimates
The total standing stock on the inner coral reef
slopes of the whole Northern Province could be
Figure 3. Mean densities of six major commercial fish families on (A) the three main zones, and (B) on the three main reef types. Vertical bars and
values in parentheses are confidence intervals at α = 0.05.
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New Caledonia: 1 – Coral reef fish
71
Figure 4. Mean biomass of six major commercial fish families on (A) the three main zones, and (B) on the three main reef types. Vertical bars and
values in parentheses are confidence intervals at α = 0.05.
estimated at 65 000 tonnes. The north zone is characterized by a very high total standing stock (43 000
tonnes), about six-fold higher than that of the east zone
(7 500). The standing stock of Acanthuridae (10 640
tonnes), Scaridae (11 350) and, to a lesser extent,
Serranidae (2 610) were particularly high in that zone
(table V). Considerable differences appeared between
standing stock of the three reef types. Total standing
stocks were approximately eight-fold higher on barrier
reefs than on both intermediate and fringing reefs
(table V). Fringing and intermediate reefs have similar
total stocks, as well as for the different families. Stocks
were particularly high on barrier reefs for Acanthuridae (14 000 tonnes) and Scaridae (12 400).
4. DISCUSSION
4.1. Between-zone patterns
Several factors may be associated with the observed
patterns, with possible combinations among them.
Terrigeneous influence may be considered as a first
factor. Several workers have mentioned terrigeneous
impact as a factor influencing the structure of fish
communities, but without any quantification/
correlation of this phenomenon [2, 51, 52]. A previous
study has clearly shown, however, that species richness, density and biomass of most fish families and/or
species were correlated to a terrigeneous runoff gradient, the highest values being found in areas far from
the coasts, although some particular species displayed
higher affinities for coastal fringing reefs [32]. The
present work strongly supports this view. The north
zone in particular experiences little terrigeneous runoff, which is in contrast with both the east and west
zones where numerous rivers and large mangrove
areas exist. In addition, the east coast is subjected to
higher and more regular rainfall when compared to the
west coast [34].
This observed pattern could also be the result of
differential availability of recruits across the lagoons,
habitat selection by recruits and differential survivorship after settlement, in combination with lagoonal
currents. These factors are likely to occur in combination, as mentioned in works in various geographical
areas [10, 30, 45]. However, as recruitment and settlement processes, and lagoonal currents in the studied
areas are as yet unknown, it is presently impossible to
estimate the contribution to overall variability from
each of these factors, either singularly or in combination.
Reef morphology, in particular the length of reefs,
may be another important factor for interpreting the
high biomass values in the north. Longer and/or larger
coral reefs usually provide a larger number of habitats
and microhabitats for fish [12, 13, 30, 46]. Indeed, the
barrier reefs in the north zone (both Cook’s and
Français’ reefs, figure 1), are very extensive and reticulated with numerous coral heads and pinnacles,
interrupted by few passes [32]. Conversely, barrier
reefs in both east and west zones are not as reticulated
and are interrupted by numerous passes.
However, the most important factor explaining the
observed patterns is likely to be linked to fishing
pressure. The whole Northern Province is characterized by very low human densities: about 41 500
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72
Y. Letourneur et al.
Table IV. Mean weight (kg), density (10–4 × indivuals·m–2) and biomass (g·m–2) of some fish species in (A) the three zones and (B) three reef types.
Values in parentheses are confidence intervals at α = 0.05 (‘+’ indicates a value < 0.1). Statistical significance was tested with SNK tests for all
pairwise comparisons among treatments (α = 0.05).
Fish species
Plectropomus leopardus
Lutjanus bohar
Lethrinus nebulosus
Cheilinus undulatus
Scarus microrhinos
Naso unicornis
Siganus lineatus
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
North
(A) Zones
West
East
Significance
1.51 (0.14)
76 (12)
11.5 (2.2)
2.31 (0.24)
38 (11)
8.7 (2.4)
1.42 (0.23)
13 (7)
1.9 (0.2)
15.03 (3.37)
11 (4)
16.8 (2.9)
1.56 (0.18)
89 (24)
13.9 (2.5)
1.36 (0.12)
129 (17)
17.5 (2.0)
0.45 (0.05)
7 (4)
0.3 (+)
1.06 (0.12)
45 (4)
4.6 (0.6)
1.73 (0.22)
17 (5)
2.8 (0.3)
0.68 (0.11)
18 (9)
1.3 (+)
8.93 (1.87)
4 (1)
3.3 (0.2)
1.37 (0.15)
50 (11)
7.3 (1.0)
1.13 (0.10)
46 (7)
5.7 (+)
0.73 (0.07)
31 (5)
2.2 (0.1)
0.89 (0.07)
42 (7)
3.8 (0.3)
1.37 (0.18)
15 (5)
2.1 (+)
0.44 (0.09)
25 (15)
1.1 (0.1)
9.01 (2.11)
3 (1)
2.7 (+)
0.99 (0.06)
49 (17)
4.8 (0.7)
0.72 (0.06)
53 (14)
3.9 (0.5)
0.21 (0.05)
15 (5)
0.9 (+)
N>W=E
N>W=E
N>W=E
N>W>E
N>W=E
N>W>E
N>W>E
E=W=N
N>W=E
N>E=W
N>E=W
N>W>E
N=W>E
N>W=E
N>W>E
N>W>E
N>E=W
N>W>E
W>N>E
W>E>N
W>E>N
Barrier
(B) Reef types
Intermediate
Fringing
Significance
1.60 (0.33)
36 (20)
5.8 (0.3)
1.95 (0.36)
40 (23)
7.8 (0.6)
0.94 (0.16)
23 (11)
2.2 (0.2)
13.41 (2.79)
11 (6)
14.1 (0.8)
1.71 (0.18)
62 (21)
10.6 (1.1)
1.25 (0.12)
108 (23)
13.5 (1.1)
0.08 (+)
2 (2)
0.1 (+)
0.99 (0.26)
66 (28)
6.5 (0.3)
1.76 (0.38)
9 (5)
1.6 (+)
0.42 (0.10)
25 (9)
1.1 (0.1)
4.91 (1.33)
2 (1)
0.9 (+)
0.96 (0.06)
69 (19)
6.7 (0.7)
0.86 (0.06)
48 (11)
4.1 (0.4)
0.56 (0.06)
20 (6)
1.1 (0.1)
0.84 (0.23)
47 (19)
3.9 (0.1)
0.52 (0.13)
7 (3)
0.4 (+)
0.35 (0.09)
30 (12)
0.8 (+)
0.79 (0.15)
1 (1)
0.1 (+)
0.99 (0.07)
35 (11)
3.5 (0.3)
0.58 (0.04)
35 (10)
2.0 (0.2)
0.68 (0.06)
40 (7)
2.7 (0.2)
B>I=F
I=F=B
I=B>F
B=I>F
B>I=F
B>I>F
B>I>F
F=I=B
B>I>F
B>I>F
B>I>F
B>I>F
B>F=I
I=B>F
B>I>F
B>I>F
B>I=F
B>I>F
F>I>B
F>I>B
F>I>B
Fish species
Plectropomus leopardus
Lutjanus bohar
Lethrinus nebulosus
Cheilinus undulatus
Scarus microrhinos
Naso unicornis
Siganus lineatus
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
weight
density
biomass
inhabitants in total, including only 900 persons in the
Bélep Islands (north zone, see figure 1). In the north
zone, only 28 tonnes of fish are caught each year,
which is negligible compared to the standing stocks
found, even though the commercialized part of these
catches (e.g. 2 tonnes versus 26 tonnes of subsistence
fishing) must be considered with caution [27]. One can
thus estimate that fish stocks in the north zone are
almost non-exploited. The high percentage of longlived species in this zone largely supports this view, in
particular because these types of species are usually
those targeted in tropical artisanal fisheries [21, 40].
Conversely, both the west and east zones are subjected
to a higher fishing pressure [27].
4.2. Between-reef type patterns
The possible causes of differences between reef
types are likely to be similar in nature to those
explaining between-zone patterns, only the particulariAquat. Living Resour. 13 (2) (2000)
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New Caledonia: 1 – Coral reef fish
73
Table V. Total standing stocks (in tonnes) and stocks of six fish families (A) in the three geographical zones and (B) in the three reef types. Values
in parentheses are confidence intervals at α = 0.05.
(A) Zones
Total
Serranidae
Lutjanidae
Lethrinidae
Scaridae
Acanthuridae
Siganidae
(B) Reef types
North
West
East
Barrier
Interm.
Fringing
43 000 (7 500)
2 610 (460)
2 040 (400)
1 350 (260)
10 640 (1 800)
11 350 (2 000)
970 (200)
14 500 (1 000)
600 (65)
1 870 (180)
580 (60)
3 280 (215)
2 640 (320)
260 (25)
7 500 (1 000)
420 (45)
690 (60)
490 (40)
2 110 (210)
1 690 (135)
300 (35)
51 700 (1 550)
3030 (510)
2 810 (500)
2 050 (350)
12 410 (1 950)
14 000 (2 840)
980 (155)
6750 (550)
340 (50)
1 160 (165)
220 (25)
2 170 (200)
970 (70)
260 (70)
6 550 (580)
260 (40)
630 (75)
160 (30)
1 450 (100)
610 (75)
290 (35)
ties of the reef types are evoked here. The large
surfaces of barrier reefs in the north zone, in combination with the high biomass in this zone, obviously
contribute significantly to the large standing stocks
found on barrier reefs. Conversely, fringing and intermediate reefs have a more patchy distribution, and
their areas are consequently smaller. In addition, the
barrier reefs are much more extensive and reticulated
than both fringing and intermediate reefs, thus likely
providing a larger number of microhabitats for
fish [12, 13, 30, 46].
As previously mentioned, there are known links
between terrestrial runoffs and the structure of fish
communities. In particular, densities and biomass of
species of commercial interest are higher on barrier
reefs subjected to low terrestrial inputs [32]. In coastal
areas, reefs located near mangroves have low fish
biomass, in relation with high turbidity [49]. However,
terrigeneous influence is not a simple factor. Indeed,
within coastal reefs, it was found that reefs subjected
to land erosion, mainly as a consequence of open fit
mining, tended to support higher densities and biomass
than reefs subjected to low terrigeneous inputs, possibly due to an increase in productivity through nutrient
enrichment [34, 39].
The observed patterns may also be linked to the
differential accessibility of the various reef types to
fishermen. Although approximately 95 % of the artisanal fishing fleet is motorized, many fishermen do not
like to fish on barrier reefs far from the coasts. This is
particularly true for the fishermen in the north zone,
where the Français’ and Cook’s reefs are far away
from the land, and in any case fish are sufficiently
abundant in areas closer. Although not frequently
observed, this fear of fishing away from the land has
also been observed in some barrier reef areas not very
far offshore (both on west and east zones) which have
numerous coral heads representing navigation hazards.
4.3. Fish stocks
When comparing fish stocks, one should remember
that the values are a function of the biomass and
surface area for which they are estimated. A survey of
the literature indicates that our findings are among the
highest biomass values known to date for coral reef
fish (table VI). The biomass found in the north zone
and on the barrier reefs was particularly high, because
of the unusually large size of most fish in these
areas [32]. As the present study was concerned only
with commercial species, the total biomass must be
even higher. Several mechanisms may explain the
observed high biomass. The possible role of the zones
sampled on overestimated findings has been discussed
elsewhere [32]: it was argued that, in New Caledonia,
the inner reef slopes (areas where most reef sampling
took place) usually support higher densities and biomass than reef flats or deeper reef areas [24]. It has
also been demonstrated that several species may be
attracted to the diver in places where human disturbance is very low, possibly inducing overestimates of
densities and/or biomass [21]. However, high biomass
values are probably explained essentially by the low
fishing pressure in the Northern Province (≈ 1 330
tonnes, [27, 32]), as found in this study. Indeed, the
densities found were not exceptionally high (table VI),
and the high biomass was mainly due to the unusually
large sizes of individuals resulting in higher mean
weights of most species, particularly those with a long
life-span.
5. CONCLUSION
Although the nature of the structure of fish communities was not really surprising, the patterns found here
are interesting in a context of an expected development of artisanal fishing activities [9, 26]. This allows
some degree of prediction of what may be found in
different zones and reef types, and consequently what
may be fished and how much [26]. Our estimates of
commercial fish standing stocks for the whole Northern Province (remember that only the inner coral reef
slopes are considered here) seem sufficiently high to
allow the development of fishing activities, in particular because the current fishing pressure, with a catch of
about 1 330 tonnes·year–1, is very low (but see the
second companion paper [27]). This is especially true
for the north zone which may be considered as
non-exploited. The standing stocks are strongly dominated by herbivorous fish, mainly Acanthuridae and
Scaridae. As these two families are currently subjected
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74
Y. Letourneur et al.
Table VI. Biomass (g·m–2) and densities (individuals·m–2) obtained on some reefs in the Indo-Pacific area (values with * considered all fish species,
including the non-commercial ones).
Area or island
New Caledonia (north)
New Caledonia (east)
New Caledonia (west)
Hawaii
Chesterfield Islands
Great Barrier Reef
Great Barrier Reef
Great Barrier Reef
Mayotte
Mayotte
Ouvéa
Ouvéa
New Caledonia
(SW lagoons)
(SW lagoons)
Réunion
Réunion
Type of biotope
Biomass
Density
Source
inner reef slopes
inner reef slopes
inner reef slopes
patch reef
fringing reef
outer reef
mid-shelf reef
fringing reef
barrier reef (reserve)
barrier reef (fished)
barrier reef
barrier reef
barrier reef (reserve)
barrier reef (reserve)
intermediate reef
rocky slope
isolated rocks
339
258
158
106*
140*
156*
237*
92*
202
79
187
260*
182
262*
301*
161*
34*
0.54
0.48
0.62
3.10*
2.60*
3.2*
8.4*
7.0*
7.51*
9.05*
0.7
3.7*
0.9
3.6*
this study
this study
this study
[4]
[24]
[52]
[52]
[52]
[29]
[29]
[24]
[24]
[25]
[25]
[25]
[31]
[31]
to low fishing pressure, one can thus imagine a
relatively easy redirection of the fishing effort towards
these fish. At the moment the fishing pressure is
mainly focused on carnivorous ‘line-fish’ species, such
as Lethrinidae, Serranidae and some non-ciguatoxic
species of Lutjanidae. As most of these fish, mainly
Lethrinidae, are not restricted to coral reefs and occur
in neighbouring areas, such as near-reef and lagoon
bottom zones [22], it is necessary to have a more
precise estimate of their total stocks. Particularly from
the perspective of local management, the only way to
allow a sustainable development of fishing activities is
to have the best possible knowledge of fish community
structure and stocks in the various biotopes forming
the lagoon (this study, [22]). This information, combined with an exhaustive analysis of the socioeconomics of the artisanal fisheries [27], should help
to prevent inappropriate management options. In this
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